VIENNA -- The 73-year-old Austrian who allegedly imprisoned his daughter for 24 years and fathered her seven children could face murder charges, prosecutors say.

Josef Fritzl told authorities in an interview that one child who died shortly after birth had no medical attention; Austrian prosecutors said that means the likelihood of a manslaughter charge, but he could be charged with "murder out of negligence."

Fritzl was to appear Friday in a closed court hearing in which he likely will have his remand custody extended for one month while the investigation continues, officials said. He could be kept for up to two years before he is formally charged.

His daughter Elisabeth, now 42 and her children, as well as Fritzl's wife Rosemarie, 68, who said she did not know of the imprisonment, were being treated in a hospital, where according to doctors they were making "remarkable progress," the Times of London said.

Meanwhile, investigators searched the Fritz home in Amstetten and archaeologists and other experts checked for other possible graves.

Col. Franz Polzer, a police spokesman, said that investigators found two previously unknown rooms in the cellar next to the dungeon where the captive family allegedly was kept.